The present invention relates generally to magnetic memory devices, and more specifically, to thermally assisted MRAM devices that provide multibit storage in the MRAM device.
Magnetoresistive random access memory (MRAM) is a non-volatile computer memory (NVRAM) technology. Unlike conventional RAM chip technologies, MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a reference magnet set to a particular polarity; the other plate's field can be changed to match that of an external field to store memory and is termed the “free magnet” or “free-layer”. This configuration is known as a magnetic tunnel junction and is the simplest structure for a MRAM bit. A memory device is built from a grid of such “cells.” In some configurations of MRAM, such as the type further discussed herein, both the reference and free layers of the magnetic tunnel junctions can be switched using an external magnetic field. In some configurations of MRAM, such as the type further discussed herein, called thermally-assisted MRAM, heat is applied to the tunnel junction when writing to a bit. In particular, the free magnet tends to be stable at a normal operating temperature, and it is more difficult to change magnetic polarity of the free magnet at a normal operating temperature. Providing heat to the free magnet may facilitate changing of a polarity of the free magnet to program a magnetic state of the free magnet. In particular, in the devices described herein, in order to thermally write the bit, a magnetic field is applied simultaneously with a heating voltage that allows for overcoming the blocking temperature of the antiferromagnetic layer, which exchange biases (providing a pinning direction) with the synthetic antiferromagnet (SAF) storage layer, allowing the storage layer magnetization to be reoriented and re-pinned by exchange bias into the new position after the device cools.